In recent years, safety-related accidents caused by lithium-ion battery (LIB) failures have often been reported and highlighted in the news. Thermal runaway (TR), as one of the most critical failure modes, and subsequent propagation can lead to catastrophic consequences for the battery pack or LIB module. In this study, TR propagation behavior between two batteries was studied. During the experiments, the TR of the first battery was triggered by mechanical abusive loading. The 3D thermal runaway model is combined with the electrical and thermal conduction model to construct a battery model for the TR model. Two typical TR propagation modes were observed and summarized from the simulation results according to different battery spacings. The mechanisms of these patterns are further discussed through the combination of computational models. High overall temperatures and localized overheating are the two main modes of TR propagation. The state of charge (SOC) is also a key factor that determines the probability and the speed of propagation. In addition, a simplified mathematical model is provided to improve the computational efficiency. Our results provide theoretical insights into the basic understanding of the TR propagation within battery packs. Results lay a strong foundation to develop an effective and efficient computing framework for the safe design of battery modules.